Archaeology is an approach to understanding lost cultures and the mute aspects of human history, without a cut-off date: in England, archaeologists have uncovered the long-lost layouts of medieval villages abandoned after the crises of the 14th century and the equally lost layouts of 17th-century parterre gardens swept away by a change in fashion. In downtown New York archaeologists have exhumed the 18th-century remains of the Black burial ground.

In the study of relatively recent cultures, which have been observed and studied by Western scholars, archaeology is closely allied with ethnography. This is the case in large parts of North America, the South Pacific, Siberia, and other places. In the study of cultures that were literate or had literate neighbours, history and archaeology supplement one another for broader understanding of the complete cultural context, as at Hadrian's Wall.

Most of human history is not described by any written records. Writing did not exist anywhere in the world until about 5000 years ago, and only spread among a relatively small number of technologically advanced civilisations. These civilisations are, not coincidentally, the best-known; they have been open to the inquiry of historians for centuries, while archaeology has arisen only recently. Even within a civilisation that is literate at some levels, many important human practices are not officially recorded. Any knowledge of the formative early years of human civilisation - the development of agriculture, cult practices of folk religion, the rise of the first cities - must come from archaeology.

Even where written records do exist, they are invariably incomplete or biased to some extent. In many societies, literacy was restricted to the elite classes, such as the clergy or the bureaucracy of court or temple. The literacy even of an aristocracy has sometimes been restricted to deeds and contracts. The interests and world-view of elites are often quite different from the lives and interests of the masses. Any writings that were produced by people more representative of the general population were unlikely to find their way into libraries and be preserved there for posterity. Thus, written records tend to reflect the biases of the literate classes, and cannot be trusted as a sole source. The material record is nearer to a fair representation of society, though it is subject to its own inaccuracies, such as sampling bias and differential preservation.

In addition to their scientific importance, archaeological remains sometimes have political significance to descendants of the people who produced them, monetary value to collectors, or simply strong aesthetic appeal. Many people identify archaeology with the recovery of such aesthetic, religious, political or economic treasures rather than the reconstruction of past societies.

This view is often espoused in works of popular fiction, such as Raiders of the Lost Ark, The Mummy, and King Solomon's Mines where the field has become profitable fodder for entertainment. When such unrealistic subjects are treated more seriously, accusations of pseudoscience are invariably levelled at their proponents (see Pseudoarchaeology, below). However, these endeavours, real and fictional, are not representative of the modern state of archaeology.

The other main division of archaeology is into historical archaeology, which examines civilisations that left behind written records and prehistoric archaeology, which concerns itself with societies that did not have writing systems. However, the term is generally valid only in Europe and Asia where literate societies emerged without colonial influence. In areas where literacy arrived relatively late, it is more convenient to use other terms to divide up the archaeological record. In areas of semi-literacy the term protohistoric archaeology can be adopted to cover the study of societies with very limited written records. One example of a protohistoric site is Fort Ross on the northern California coast, which included settlements of literate Russians and non-literate American Indians and Alaska nativess.

Ethnoarchaeology is the study of modern societies resembling extinct ones of archaeological interest, for archaeological purposes. It is often difficult to infer solid conclusions about the structure and values of ancient societies from their material remains, not only because objects are mute and say little about those who crafted and used them, but also because not all objects survive to be uncovered by scholars of a later age. Ethnoarchaeology seeks to determine, for instance, what kinds of objects used in a living settlement are deposited in middens or other places where they may be preserved, and how likely an object is to be discarded near to the place where it was used.

Taphonomy is the study of how objects decay and degrade over time. This information is critical to interpretation of artefacts and other objects, so that the work of ancient people can be differentiated from the later work of living creatures and elemental forces.

A selective list of sub-disciplines distinguished by time period or region of study is given below.

Cultural resources management (CRM) (also called heritage management in Britain) is a branch of archaeology that accounts for most research done in the United States and much of that in western Europe as well. In the United States, CRM archaeology has been a growing concern since the passage of the National Historic Preservation Act of 1966 and most of the archaeology done in that country today proceeds from either direct or related requirements of that measure. In the United States, the vast majority of taxpayers, scholars, and politicians believe that CRM has helped to preserve much of that nation's history and prehistory that would have otherwise been lost in the expansion of cities, dams, and highways. Along with other statutes, this mandates that no construction project on public land or involving public funds may damage an unstudied archaeological site.

The application of CRM in the United Kingdom is not limited to government-funded projects. Since 1990 PPG 16 has required planners to consider archaeology as a material consideration in determining applications for new development. As a result, numerous archaeological organisations undertake mitigation work in advance of (or during) construction work in archaeologically sensitive areas, at the developer's expense.

Among the goals of CRM are the identification, preservation, and maintenance of cultural sites on public and private lands, and the removal of culturally valuable materials from areas where they would otherwise be destroyed by human activity, such as proposed construction. This study involves at least a cursory examination to determine whether or not any significant archaeological sites are present in the area affected by the proposed construction. If these do exist, time and money must be allotted for their excavation. If initial survey and/or test excavation indicates the presence of an extraordinarily valuable site, the construction may be prohibited entirely. CRM is a thriving entity, especially in the United States and Europe where archaeologists from private companies and all levels of government engage in the practice of their discipline.

Cultural resources management has doubtless mitigated the destruction of the archaeological record by the ever-sprawling works of Western civilisation, but it leaves something to be desired. CRM is conducted by private companies that bid for projects by submitting proposals outlining the work to be done and an expected budget. It is not unheard-of for the agency responsible for the construction to simply choose the proposal that asks for the least funding. CRM archaeologists face considerable time pressure, often being forced to complete their work in a fraction of the time that might be allotted for a purely scholarly endeavour.

Survey work has many benefits if performed as a preliminary exercise to, or even in place of, excavation. It requires relatively little time and expense, because it does not require processing large volumes of soil to search out artefacts. (Nevertheless, surveying a large region or site can be expensive, so archaeologists often employ sampling methods.) It avoids ethical issues (of particular concern to descendant peoples) associated with destroying a site through excavation. It is the only way to gather some forms of information, such as settlement patterns and settlement structure. Survey data are commonly assembled into maps, which may show surface features and/or artefact distribution.

The simplest survey technique is surface survey. It involves combing an area, usually on foot but sometimes with the use of mechanised transport, to search for features or artefacts visible on the surface. Surface survey cannot detect sites or features that are completely buried under earth, or overgrown with vegetation. Surface survey may also include mini-excavation techniques such as augers, corers, and shovel test pits.

Aerial survey is conducted using cameras attached to aircraft, balloons or even kites. A bird's-eye view is useful for quick mapping of large or complex sites. Aerial imaging can also detect many things not visible from the surface. Plants growing above a stone structure, such as a wall, will develop more slowly, while those above other types of features (such as middens) may develop more rapidly. Photographs of ripening grain, which changes colour rapidly at maturation, have revealed buried structures with great precision. Aerial survey also employs infrared, ground-penetrating radar wavelengths, and thermography.

Geophysical survey is the most effective way to see beneath the ground. Magnetometers detect minute deviations in the Earth's magnetic field caused by iron artefacts, kilns, some types of stone structures, and even ditches and middens. Devices that measure the electrical resistivity of the soil are also widely used. Most soils are moist below the surface, which gives them a relatively low resistivity. Features such as hard-packed floors or concentrations of stone have a higher resistivity.

Archaeological excavation existed when the field was still the domain of amateurs, and it remains the source of the majority of data recovered in most field projects. It can reveal several types of information usually not accessible to survey, such as stratigraphy, three-dimensional structure, and verifiably primary context.

Modern excavation techniques require that the precise locations of objects and features, known as their provenance or provenience, be recorded. This always involves determining their horizontal locations, and sometimes vertical position as well. Similarly their association, or relationship with nearby objects and features, needs to be recorded for later analysis. This allows the archaeologist to deduce what artefacts and features were likely used together and which may be from different phases of activity. For example, excavation of a site reveals its stratigraphy; if a site was occupied by a succession of distinct cultures, artefacts from more recent cultures will lie above those from more ancient cultures.

Excavation is the most expensive phase of archaeological research. Also, as a destructive process, it carries ethical concerns. As a result, very few sites are excavated in their entirety. Sampling is even more important in excavation than in survey. It is common for large mechanical equipment, such as backhoes (JCBs), to be used in excavation, especially to remove the topsoil (overburden), though this method is increasingly used with great caution. Following this it is usual to hand-clean the exposed area with trowels or hoes to ensure that all features are apparent.

The next task is to produce a site plan and then use it to help decide the method of excavation. Features dug into the natural subsoil are normally excavated in portions in order to produce a visible archaeological section for recording. Scaled plans and sections of individual features are all drawn on site, black and white and colour photographs of them are taken and recording sheets are filled in describing the context of each. All this information serves as a permanent record of the now-destroyed archaeology and is used in describing and interpreting the site.

The history of archaeology has been one of increasing professionalisation, and the use of an increasing range of techniques, to obtain as much data on the site being examined as possible.

Excavations of ancient monuments and the collection of antiquities have been taking place for thousands of years, but these were mostly for the extraction of valuable or aesthetically pleasing artefacts.

The first major phase in the history of archaeological theory is commonly referred to as cultural, or culture history. The product of cultural history was to group sites into distinct "cultures", to determine the geographic spread and time span of these cultures, and to reconstruct the interactions and flow of ideas between them. Cultural history, as the name suggests, was closely allied with the science of history. Cultural historians employed the normative model of culture, the principle that each culture is a set of norms governing human behaviour. Thus, cultures can be distinguished by patterns of craftsmanship; for instance, if one excavated sherd of pottery is decorated with a triangular pattern, and another sherd with a chequered pattern, they likely belong to different cultures. Such an approach naturally leads to a view of the past as a collection of different populations, classified by their differences and by their influences on each other. Changes in behaviour could be explained by diffusion whereby new ideas moved, through social and economic ties, from one culture to another.

The Australian archaeologist Vere Gordon Childe was one of the first to explore and expand this concept of the relationships between cultures especially in the context of prehistoric Europe. By the 1920s sufficient archaeological material had been excavated and studied to suggest that diffusionism was not the only mechanism through which change occurred. Influenced by the political upheaval of the inter-war period Childe then argued that revolutions had wrought major changes in past societies. He conjectured a Neolithic Revolution, which inspired people to settle and farm rather than hunt nomadically. This would have led to considerable changes in social organisation, which Childe argued led to a second Urban Revolution that created the first cities. Such macro-scale thinking was in itself revolutionary and Childe's ideas are still widely admired and respected.

In the 1960s, a number of young, primarily American archaeologists, such as Lewis Binford, rebelled against the paradigms of cultural history. They proposed a "New Archaeology", which would be more "scientific" and "anthropological". They came to see culture as a set of behavioural processes and traditions. (In time, this view gave rise to the term processual archaeology). Processualists borrowed from the exact sciences the idea of hypothesis testing and the scientific method. They believed that an archaeologist should develop one or more hypotheses about a culture under study, and conduct excavations with the intention of testing these hypotheses against fresh evidence. They had also become frustrated with the older generation's teachings through which cultures had taken precedence over the people being studied themselves. It was becoming clear, largely through the evidence of anthropology, that ethnic groups and their development were not always entirely congruent with the cultures in the archaeological record.

In the 1980s, a new movement arose led by the British archaeologists Michael Shanks, Christopher Tilley Daniel Miller and Ian Hodder. It questioned processualism's appeals to science and impartiality by claiming that every archaeologist is in fact biased by his or her personal experience and background, and thus truly scientific archaeological work is difficult or impossible. This is especially true in archaeology where experiments (excavations) cannot possibly be repeatable by others as the scientific method dictates. Exponents of this relativistic method, called post-processual archaeology, analysed not only the material remains they excavated, but also themselves, their attitudes and opinions. The different approaches to archaeological evidence which every person brings to his or her interpretation result in different constructs of the past for each individual. The benefit of this approach has been recognised in such fields as visitor interpretation, cultural resource management and ethics in archaeology as well as fieldwork. It has also been seen to have parallels with culture history. Processualists critique it, however, as without scientfic merit. Even if you can't perfectly replicate digs, one should try to follow science as rigorously as possible, they say.

Post-processualism provided an umbrella for all those who decried the processual model of culture, which many feminist and neo-Marxist archaeologists for example believed treated people as mindless automatons and ignored their individuality.

This divergence of archaeological theory has not progressed identically in all parts of the world where archaeology is conducted. Australian archaeologists have embraced post-processualism, while those in the United States freely combine it with older approaches and methods.

Much thorough and productive research has indeed been conducted in dramatic locales such as Copán and the Valley of the Kings, but the stuff of modern archaeology is not so reliably sensational. In addition, archaeological adventure stories tend to ignore the painstaking work involved in modern survey, excavation and data processing techniques. Some archaeologists refer to such portrayals as 'pseudoarchaeology'.

Nevertheless, archaeology has profited from its portrayal in the mainstream media. Many practitioners point to the childhood excitement of Indiana Jones films and Tomb Raider games as the inspiration for them to enter the field. Archaeologists are also very much reliant on public support, the question of exactly who they are doing their work for is often discussed. Without a strong public interest in the subject, often sparked by significant finds and celebrity archaeologists, it would be a great deal harder for archaeologists to gain the political and financial support they require.

Where possible, archaeologists now make more provision for public involvement and outreach in larger projects than they once did. However, the move towards being more professional has meant that volunteer places are now relegated to unskilled labour, and even this is less freely available than before. Developer-funded excavation necessitates a well-trained staff that can work quickly and accurately, observing the necessary Health and Safety and indemnity insurance issues involved in working on a modern building site to tight deadlines. Certain charities and local government bodies sometimes offer places on research projects either as part of academic work or as a defined community project. There is also a flourishing industry selling places on commercial training excavations and archaeological holiday tours.

Archaeologists prize local knowledge and often liaise with local historical and archaeological societies. Anyone looking to get involved in the field without having to pay for the privilege should contact a local group.

Pseudoarchaeology is an umbrella term for all activities that claim to be archaeological but in fact violate commonly accepted archaeological practices. It includes much fictional archaeological work (discussed above), as well as some actual activity. Many non-fiction authors have ignored the scientific methods of processual archaeology, or the specific critiques of it contained in Post-processualism.

An example of this type is the author, Erich von Däniken. His Chariots of the Gods (1968), together with many subsequent, lesser-known works, expounds a theory of ancient contacts between human civilisation on Earth and more technologically advanced extraterrestrial civilisations. (This theory, known as palaeocontact theory, is not exclusively Däniken's.) Works of this nature are usually marked by the renunciation of well-established theories on the basis of limited evidence, and the interpretation of evidence with a preconceived theory in mind.

Archaeologists have also been obliged to re-examine what constitutes an archaeological site in view of what native peoples believe to constitute sacred space. To many native peoples, natural features such as lakes, mountains or even individual trees have cultural significance. Australian archaeologists especially have explored this issue and attempted to survey these sites in order to give them some protection from being developed. Such work requires close links and trust between archaeologists and the people they are trying to help and at the same time study.

While this cooperation presents a new set of challenges and hurdles to fieldwork, it has benefits for all parties involved. Tribal elders cooperating with archaeologists can prevent the excavation of areas of sites that they consider sacred, while the archaeologists gain the elders' aid in interpreting their finds. There have also been active efforts to recruit aboriginal peoples directly into the archaeological profession.